1. Field of the Invention
The present invention relates to an injection molding machine and more particularly to a controller of an injection molding machine for performing accurate measuring of resin.
2. Description of Related Art
In an in-line screw type injection molding machine, a screw in a heating cylinder is rotated to melt and knead a resin material, and the molten resin is fed to the distal end portion of the heating cylinder under pressure. As this is done, the screw is retreated under pressure control. When the screw reaches a set measuring point, its rotation and retreat are stopped, and the amount of the resin is measured. Likewise, in a pre-plasticization type injection molding machine, a molten resin is fed to the distal end portion of a cylinder, whereupon a plunger is retreated by the pressure of the fed resin. When the plunger reaches a set position, the amount of the resin is measured. After this measurement, the screw or the plunger is advanced to inject and fill the molten resin into a mold.
Dispersion of the fill of the molten resin in the mold should be reduced in order to improve the quality of molded products.
Injection is performed after the screw or the plunger is retreated to a measuring point or position. If the measured molten resin is directly injected and filled into the mold as this is done, an accurate fill can be obtained, so that the molded products are not subject to dispersion in weight and their quality can be kept uniform. However, there is a problem that the molten resin flows backward to change the amount of the measured resin as an injection process is started on completion of the measurement, so that an accurate amount cannot be injected.
A valve, such as a check valve or a check ring, is attached to the distal end of the screw, whereby the molten resin produced by the rotation of the screw is fed to the distal end portion of the cylinder in the measuring process, and the molten resin at the distal end portion of the screw is injected into the mold without flowing backward in the injection process. This valve prevents the molten resin from moving between the distal and proximal end portions of the screw and changing the measured resin amount during the time period from the completion of measurement to the start of injection. Thus, the measured amount of the resin can be injected accurately. However, there is a problem that the molten resin may be caused to flow backward by delayed operation of backflow preventing means, such as a valve for preventing backflow of the molten resin, so that an accurate amount of molten resin can be injected. Various techniques are proposed to improve this problem.
In a pre-plasticization type injection molding machine according to a known invention (see FIG. 1 of JP 2-147312A), a plunger is retreated on completion of measurement so that the pressure of a molten material at the distal end portion of a screw is zero. Thereafter, the plunger is advanced to retreat a check valve, thereby closing a resin passage that extends from the proximal end portion of a cylinder to the distal end portion thereof. Thus, the molten material is prevented from flowing backward, so that dispersion of the measured resin amount is removed.
In a known in-line screw type injection molding machine (see Paragraph [0031] of JP 11-240052A), a screw is stopped from rotating when it is retreated to a measuring point. After the retreat of the screw is stopped, the screw is reversely rotated in its stopped position, and the pressure of a resin at the rear end portion of a cylinder is lowered to retreat a check valve, thereby closing a resin passage. Thereafter, the resin is sucked back and injection is then started, whereby dispersion of the measured resin amount is removed.
In another known case (see Claims 1 and 2 of JP 3118188B), a screw is kept in position as it is reversely rotated in a suck-back process on completion of measurement, and injection is started thereafter. In still another known case (see Effect of the Invention of JP 2-38381B), a screw is reversely rotated and retreated at the same time on completion of measurement, and injection is started thereafter. In a further known case (see Paragraphs [0015], [0016] and [0017] of JP 9-29794B), a screw is reversely rotated after a resin is sucked back on completion of measurement, and a resin passage is closed by a ring valve. Thereafter, injection is performed after the screw is advanced to a position before the start of the suck-back process and temporarily stopped. Alternatively, injection is performed after the screw is reversely rotated as it is advanced to the position before the start of the suck-back process and temporarily stopped after the resin is sucked back. In another known case (see Claims 1 and 2 of JP 60-76321A), preliminary injection is performed after measurement in order to improve the accuracy of injection. In this preliminary injection, a pressure lower than the pressure of injection of melted oil into a cavity is maintained for a fixed period of time after measurement or so that it increases to a fixed pressure. After a check ring is closed, the position concerned is stored in memory, and injection is performed for a fixed distance from the stored position. Alternatively, a screw is reversely rotated during the preliminary injection. In another known case (see claims of JP 53-39358A), a screw is simply pressurized and advanced to and stopped at a predetermined position on completion of measurement, and injection is performed in this position as a reference point. In another conventionally known case (see claims of JP 52-151352A), a screw is subjected to a pressure for a fixed period of time such that a check valve is moved to close a resin passage securely on completion of measurement, it is stopped at a point where a predetermined pressure is reached, and injection is performed in this position as a reference point.
In an alternative case (see Paragraph [0011] of JP 3652681B), a surplus measured resin that is produced by excessive screw rotation after measurement is returned by reverse rotation. In another alternative case (see Paragraph [0011] of JP 2004-154994A), reverse rotation of a screw is started immediately before the end of measurement, and the screw is stopped at an accurate measurement end position.
According to the prior art techniques described above, backflow or the like of the molten resin at the distal end portion of the cylinder is prevented from occurring and changing the measured resin amount during injection.
In a known control method for obtaining an acute measured resin amount (see the last line of column 6 to line 21 of column 7 of JP 1-26857B), a plurality of sets of back pressure commands and screw rotational frequency commands are provided so that combinations of back pressures and screw rotational frequencies can be controlled in association with one another. In this case, the measurement is made uniform by controlling the screw rotational frequencies and screw retreating speeds to be zero at the end of a measuring process.
In another known method (see claims and FIG. 2 of JP 64-6931B), moreover, a stable measured resin amount is obtained by gradually reducing screw rotational speeds used before the stop of measurement based on screw retreat positions.
In the pre-plasticization type injection molding machine described in JP 2-147312A, the plunger is retreated or advanced after completion of measurement. In the injection molding machines described in JP 11-240052A, JP 3118188B, JP 9-29794B and JP 3652681B, moreover, an accurate measured amount (injection amount) is obtained by separately performing the reverse rotation and the advance or retreat of the screw after the end of measurement. However, the measuring operation based on these prior art techniques are disadvantageous in that its cycle time is inevitably extended, as compared with measuring operation of a pre-plasticization type injection molding machine in which the plunger is neither advanced nor retreated or a conventional in-line screw type injection molding machine in which the screw is not reversely rotated.
According to the technique described in JP 2-38381B in which the screw is reversely rotated as it is retreated after completion of measurement, moreover, the internal pressure of the screw is fully lowered if the screw is reversely rotated a little, although there is no possibility of an unfavorable extension of the cycle time. Therefore, the screw must be reversely rotated to the minimum necessary extent for precise molding, which may possibly result in excessive reverse rotation. If the screw is excessively reversely rotated, however, the molten resin is caused to flow backward and excessively filled into a groove of the screw, inevitably causing dispersion of the measurement in the next molding cycle. If the retreat distance of the screw is shortened in order to reduce the reverse rotation of the-screw, moreover, decompression by the screw retreat is so small that molding is destabilized by dripping from a nozzle. If the retreating speed of the screw in reverse rotation is too high, furthermore, a pressure reducing effect of the measured resin by the reverse rotation of the screw cooperates with a pressure reducing effect of the measured resin by the screw retreat. Thereupon, a sudden pressure change occurs, so that the measured amount of the resin changes inevitably. This increases dispersion in the injection pressure or the minimum cushion amount, and its control is very difficult.